We here in NY/NJ install the Gear 300 across the board for our 86 customers. We have a relatively large sample size and go through probably 300-400 gallons of Gear 300 a year.
With that said, using the Gear 300 75w90 in both our own street cars as well as our heavily tracked BRZ we have seen no issues with it and the advantage is that it is both a GL4 and GL5 so you can use the same fluid in both trans and rear diffy. If you use the LS version make sure you don't inadvertently put that into the Trans as it is not GL4 rated.
Our service interval is 30k miles for our clients and yearly for our track vehicles. Given the stop and go nature of our driving here in NY/NJ the heat buildup in the summers is probably close to or in excess of what you will see in the Southwest.
Hope that helps. We also sell 4 Liters of Motul Gear 300 for $85 shipped, just shoot us an e-mail or PM.
-Mike Paisan
Maintaining, Modifying and Educating TriState Enthusiasts since 2001.
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